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Dasymys incomtusAfrican marsh rat

By Thomas Herrmann

Geographic Range

African marsh rats (Dasymys incomtus) are found throughout sub-Saharan Africa in the following countries: Angola, Democratic Republic of the Congo, Ethiopia, Kenya, Malawi, South Africa, South Sudan, Tanzania, Uganda, Zambia, and Zimbabwe. Therefore their range occurs entirely within the Ethiopian bioregion. There is some uncertainty regarding the range of African marsh rats because they are rarely captured. Many current estimates of their range may be inaccurate because they do not account for the habitat loss and fragmentation that they have faced due to the destruction of wetlands. African marsh rats live in areas that are generally wet and have dense vegetation as ground cover. They are most commonly found in wetland environments, but can be found anywhere with large amounts of annual rainfall - usually about 500 mm. Fluctuations in climate in the past may be responsible for the reduced range of African marsh rats today, and can also explain the discontinuous nature of their population (Mullin et al., 2002). (Mullin, et al., 2002)

Habitat

The range of African marsh rats is restricted to wetland areas or swamps with tall grasses. They also inhabit riverbeds that occasionally get flooded. African marsh rats are habitat specialists and require wetlands to survive. Their dependence on wetlands can cause them to be threatened by droughts, or the destruction of wetlands (Mullin et al., 2002). They can most typically be found in reed beds, or areas with semi-aquatic grasses (Skinner and Chimimba, 2005). (Mullin, et al., 2002; Skinner and Chimimba, 2005)

  • Range elevation
    0 to 1500 m
    0.00 to 4921.26 ft

Physical Description

African marsh rats are medium-sized rodents with an average mass of 118 to 171 g. Their bodies range in length from 153 to 175 mm, with tails of similar length to their bodies, ranging from 110 to 175 mm. African marsh rats have long feet that are 27 to 39 mm long and medium sized ears 16.5 to 20.5 mm long. African marsh rats are dark brown in color. They have small snouts and long, hairless tails (Mullin et al., 2002). Some reports of African marsh rats say that the pelage on their bodies is a dark grey or black color, with an even darker head and lighter grey cheeks and flanks. Their undersides have white fur, while the hair on their backs is distinctly shiny (Skinner and Chimimba, 2005). (Mullin, et al., 2002; Skinner and Chimimba, 2005)

  • Sexual Dimorphism
  • sexes alike
  • Range mass
    118 to 171 g
    4.16 to 6.03 oz
  • Range length
    153 to 175 mm
    6.02 to 6.89 in

Reproduction

African marsh rats breed in a specific breeding season that occurs during periods of high rainfall. They pair with mates to produce offspring. It is unknown if they are monogamous or polygamous, but for the purpose of a lab experiment they were paired with only one other individual for the whole experiment (Pillay, 2003). Other rats of the same subfamily typically display promiscuous mating systems, so it is possible that African marsh rats are also polygynandrous (Nowak, 1999). (Nowak, 1999; Pillay, 2003)

African marsh rats produce smaller litters than most other rodents, with an average litter size of only 2.9. Newborns are also smaller relative to adults. Offspring only weigh about 5% of the mass of adults, on average. Offspring reach sexual maturity at 9 weeks of age for males and 17 weeks for females. Male African marsh rats weigh 98 g at maturity, while females weigh 136 g at maturity. This delayed maturity in females is not common in similar rodent species, who commonly reach maturity earlier than 17 weeks after birth. The time to maturity for female African marsh rats is entirely age dependent, since altering their environment does not change their age at maturity. The reasons for this delay are unknown. African marsh rats have a very short time between weaning young of one litter and giving birth to another. Young are weaned after about 24 days and have been known to produce a new litter on an interval of 32 days (Pillay, 2003). They also live in isolated groups and experience low amounts of gene flow between populations (Mullin et al., 2002). (Mullin, et al., 2002; Pillay, 2003)

  • Breeding interval
    African marsh rats breed every 32 days during the wet season.
  • Breeding season
    August to December
  • Average number of offspring
    2.9
  • Average gestation period
    29 days
  • Average weaning age
    24 days
  • Average age at sexual or reproductive maturity (female)
    17 weeks
  • Average age at sexual or reproductive maturity (male)
    9 weeks

Newborn African marsh rats are altricial, meaning that, after birth, the young must be cared for by their parents until they are developed enough to be weaned. Newborns do not open their eyes until 12 to 14 days after birth and weaning occurs after 24 days. This time spent caring for offspring represents a large amount of parental investment by African marsh rat parents, that is representative of a K-selected species. The smaller litter size of African marsh rats also allows mothers to invest more in each individual offspring (Pillay, 2003). (Pillay, 2003)

Lifespan/Longevity

African marsh rats have an unknown lifespan. None have been kept in captivity long enough to learn their lifespan, and the difficulty of finding them in the wild has made it difficult to understand. Typically, other old world rats live between a couple months to a year or two in the wild, but it is unknown if African marsh rats follow these patterns (Nowak, 1999). (Nowak, 1999)

Behavior

Little is known about the behavior of African marsh rats due to the difficulty in capturing live specimens, and their tendency to live in small, isolated groups (Mullin et al., 2002). African marsh rats are known to build complex nests in order to remain protected while weaning their young (Pillay, 2003). They are primarily nocturnal, although some have been observed during the day. They are semi-aquatic and typically build their burrows along the banks of rivers either running parallel to the river or directly towards it. African marsh rats are able to swim well and are able to swim through their burrows, even when they are inundated with some water (Skinner and Chimimba, 2005). (Mullin, et al., 2002; Pillay, 2003; Skinner and Chimimba, 2005)

Communication and Perception

Little is known about the communication techniques used by African marsh rats, due to the rarity of seeing multiple individuals of the species interact with one another. Typically, other species in the subfamily Murinae use combinations of vision, smell, hearing, touch, and chemical traces to communicate and navigate their surroundings. Due to the fact that they are primarily nocturnal, it is likely that African marsh rats typically rely on hearing and touch in order to navigate in the darkness (Carleton and Musser, 1984). (Carleton and Musser, 1984)

Food Habits

The diet of African marsh rats is primarily made up of green plant material. However, some specimens have been seen with some insects in their stomach contents as well. They are primarily herbivorous though (Carleton and Martinez, 1991). They typically rely on the stems and heads of semi-aquatic grasses and reeds that occur around their burrows (Skinner and Chimimba, 2005). (Carleton and Martinez, 1991; Skinner and Chimimba, 2005)

  • Animal Foods
  • insects
  • Plant Foods
  • wood, bark, or stems

Predation

Little is known about the predation of African marsh rats due to their rarity, as well as their difficulty to find and capture. However, the necessity of dense ground vegetation for African marsh rats to survive suggests that they create runways and nest sites hidden in the grass to escape predators in their area (Carleton and Martinez, 1991). Other rats in Africa face predation from larger mammals such as jackals, wild cats, and leopards (Kingdon et al., 2011). (Carleton and Martinez, 1991; Kingdon, et al., 2011)

Ecosystem Roles

African marsh rats are primary consumers, feeding on grasses and stems of plants found in wetlands and swamps (Casanovas-Vilar et al., 2011). They can also be considered an ecosystem engineer, due to their tendency to build complex nests and burrows (Pillay, 2003). (Casanovas-Vilar, et al., 2011; Pillay, 2003)

Economic Importance for Humans: Positive

Historically, African marsh rats have no economic importance for humans, due to the rarity of sightings, as well as the difficulty of trapping them.

Economic Importance for Humans: Negative

Due to their lack of common interaction with humans, little is known about the negative impacts of African marsh rats on humans. There are no known significant negative economic impacts.

Conservation Status

According to the IUCN red list, African marsh rats are listed as least concern with an unknown population trend (Taylor, 2016). They are listed as vulnerable on the regional red list organized by the Endangered Wildlife Trust, as well as the South African National Biodiversity Institute. The main reason for this listing is the destruction of wetland and swamp habitat to create farmland (Pillay et al., 2016). (Pillay, et al., 2016; Taylor, 2016)

Contributors

Thomas Herrmann (author), University of Washington, Laura Prugh (editor), University of Washington, Galen Burrell (editor), Special Projects.

Glossary

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

carnivore

an animal that mainly eats meat

chemical

uses smells or other chemicals to communicate

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

female parental care

parental care is carried out by females

folivore

an animal that mainly eats leaves.

fossorial

Referring to a burrowing life-style or behavior, specialized for digging or burrowing.

herbivore

An animal that eats mainly plants or parts of plants.

insectivore

An animal that eats mainly insects or spiders.

marsh

marshes are wetland areas often dominated by grasses and reeds.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

nocturnal

active during the night

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

soil aeration

digs and breaks up soil so air and water can get in

swamp

a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.

tactile

uses touch to communicate

terrestrial

Living on the ground.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

Carleton, M., C. Martinez. 1991. Morphometric Differentiation among West African Populations of the Rodent Genus Dasymys (Muroidea, Murinae), and its Taxonomic Implications.. Proceedings of the Biological Society of Washington., 104(3): 419-435. Accessed July 06, 2019 at https://repository.si.edu/bitstream/handle/10088/18663/vz_PBSW104_3_419_435.pdf?sequence=1&isAllowed=y.

Carleton, M., G. Musser. 1984. Orders and Families of Recent Mammals of the World.. New York: John Wiley and Sons.

Casanovas-Vilar, I., J. Van Dam, S. Moyà-Solà, L. Rook. 2011. Late Miocene insular mice from the Tusco-Sardinian palaeobioprovince provide new insights on the palaeoecology of the Oreopithecus faunas. Journal of Human Evolution, 61(1): 42-49.

Kingdon, J., B. Agwanda, M. Kinnaird, T. O`Brien, C. Holland, T. Gheysens, F. Vollrath, M. Boulet-Audet. 2011. A poisonous surprise under the coat of the African crested rat. Proceedings of the Royal Society B: Biological Sciences, 279: 675-680.

Mullin, S., N. Pillay, P. Taylor, G. Campbell. 2002. Genetic and morphometric variation in populations of South African Dasymys incomtus incomtus (Rodentia, Murinae). Mammalia, 66(3): 381-404.

Nowak, R. 1999. Walker's Mammals of the World, vol. 2. Baltimore and London: The Johns Hopkins University Press.

Pillay, N. 2003. Reproductive biology of a rare African rodent, the water rat, Dasymys incomtus. Journal of Mammalogy, 84(2): 505-512.

Pillay, N., P. Taylor, R. Baxter, D. Jewitt, G. Pence, M. Child. 2016. "A conservation assessment of Dasymys spp." (On-line). Accessed July 06, 2019 at https://www.ewt.org.za/wp-content/uploads/2019/02/9.-African-Marsh-Rat-Dasymys-spp_VU.pdf.

Skinner, J., C. Chimimba. 2005. The mammals of the southern African sub-region. Cambridge University Press.

Taylor, P. 2016. "Dasymys incomtus" (On-line). The IUCN Red List of Threatened Species. Accessed July 06, 2019 at http://dx.doi.org/10.2305/IUCN.UK.2016-3.RLTS.T6269A22436584.en..

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